Abstract: Background  Endolymphatic sac tumor (ELST) is a rare tumor originating fromendolymphatic epithelium of inner ear. This tumor exhibits low-grade malignancy with benign histopathological appearance and clinically destructive behavior which occurs in the skull base and frequently invades the posterior petrous bone, the mastoid, semicircular canal, cerebellopontine angle structures and cranial nerve. The presence of intracranial ELST always makes the diagnosis challenge for clinicians and pathologists. Herein we describe a case of ELST in skull base. The clinicopathology of this tumor and its differential diagnosis are discussed. Methods  The clinical manifestation of a patient with primary ELST occurring in right cerebellopontine angle was presented retrospectively. Resected mass was routinely paraffin-embedded and stained with hematoxylin and eosin. Dako EnVision immunohistochemical staining system was used to detect the tumor antigen expressions, including cytokeratin (CK), vimentin (Vim), epithelial membrane antigen (EMA), carcinoembryonic antigen (CEA), synaptophysin (Syn), chromogranin A (CgA), S-100 protein (S-100), glial fibrillary acidic protein (GFAP), thyroglobulin (TG), thyroid transcription factor-1 (TTF-1) and Ki-67. Results  A 32-year-old male patient presented with 20-year history of progressive hearing loss. MRI scan revealed an expansile lytic lesion of the mastoid process of the right petrous bone, measuring 4.20 cm × 3.30 cm × 2.00 cm, occupied the right cerebellopontine angle with infiltration of surrounding dura mater. But the lesion did not break the dura mater and invade the brain parenchyma. Craniotomy was performed and the tumor was removed totally. Histological examination revealed a papillary, cystic or glandular architecture in mass. The papillary and glandular structures were lined by a single layer of flattened cuboidal-to-columnar cells. The stroma of the papillary fronds was richly vascularized and chronically inflamed. There were minimal cellular pleomorphism and rare mitotic activity and necrosis was not observed. There were cystic glandular spaces filled with colloid-like material which was remarkably similar to thyroid tissue. Immunohistochemical staining showed that the tumor cells were diffusely positive for CK (AE1/AE3), Vim, EMA and focally positive for CEA, but negative for Syn, CgA, S-100, GFAP, TG and TTF-1. The Ki-67 index was less than 1%. The colloid-like material showed positive reactivity with periodic acid-Schiff (PAS). Based on clinical presentation and histological findings, a final histological diagnosis of primary papillary ELST in cerebellopontine angle was made according to the criteria of WHO classification. The patient had not received radiotherapy or chemotherapy and attended follow-up for 3 months, without any neurological deficit or signs of recurrence. Conclusion  ELST is rare skull base tumor originating from endolymphatic epithelium within the vestibular aqueduct, characterized clinically by slow growth with local invasion and bone destruction. Due to the rarity of this tumor, it can easily be confused with other tumors of skull base with papillary architecture such as chorioid plexus papilloma, papillary ependymoma, papillary meningioma, middle ear adenoma, and metastatic papillary carcinoma of the thyroid. Complete excision is the best guarantee against tumor recurrence, while recurrence may occur due to subtotal resection.

Key words: Endolymphatic sac, Adenocarcinoma, papillary, Ear, inner, Immunohistochemistry

摘要： 研究背景 乳头状内淋巴囊肿瘤是起源于内耳内淋巴囊的低度恶性肿瘤，临床罕见，因其进展过程缓慢、病变位置深、术前难以明确诊断而易延误病情，给临床诊断与治疗带来一定困难。本文重点探讨原发于颅内脑桥小脑角区乳头状内淋巴囊肿瘤的病理学特征，以期提高临床的重视程度。方法 回顾1 例原发于右侧脑桥小脑角的乳头状内淋巴囊肿瘤患者的临床表现，通过HE 染色、免疫组织化学染色和特殊染色分析其临床病理特点及免疫表型。结果 男性患者，32 岁。近20 年来进行性右耳听力下降并头痛。CT 检查可见右侧岩骨乳突部膨胀性生长的溶骨性病灶；MRI 显示肿物约4.20 cm ×3.30 cm × 2.00 cm 大小，T₂WI呈稍高信号，增强后肿瘤灶不均匀强化。肿物破坏右侧岩骨并侵入乳突，累及邻近硬脑膜，但未突破硬脑膜侵犯脑实质。术中可见肿瘤位于右侧脑桥小脑角区，侵袭破坏岩骨并累及硬脑膜，但第四脑室侧孔未受累。肿瘤由乳头状、囊状或腺样结构组成，呈浸润性生长，破坏周围骨质；单层扁平和立方上皮细胞覆盖，乳头间质为较丰富的血管轴心和少量浸润的炎性细胞，肿瘤细胞异型性不明显，胞质空亮或嗜酸性，胞核呈不规则圆形，未见核分裂象，腺腔内还可见嗜酸性胶样物质，类似甲状腺滤泡。免疫组织化学染色肿瘤细胞弥漫表达广谱细胞角蛋白、波形蛋白和上皮膜抗原，灶性表达癌胚抗原，但不表达突触素、神经元特异性烯醇化酶、S-100 蛋白、甲状腺球蛋白和甲状腺转录因子-1，Ki-67 抗原标记指数< 1%。特殊染色腺腔内嗜酸性胶样物质过碘酸-雪夫染色阳性。病理诊断为右侧脑桥小脑角区乳头状内淋巴囊肿瘤。结论 内淋巴囊肿瘤为临床罕见低度恶性腺癌，起源于内耳内淋巴囊上皮细胞，尽管其组织形态表现温和、生长缓慢，但可侵袭破坏周围组织。诊断颅底乳头状病变时应提高警惕，并注意与其他具有相似组织学结构的颅内肿瘤相鉴别。

关键词: 内淋巴囊, 腺癌, 乳头状, 内耳, 免疫组织化学